I understand what you are saying but I am going to have to respectfully disagree. Jack Del Rio played with John Randle under Tony Dungy. And before that with Jimmy Johnson and the Dallas Cowboys. He is fully aware of how dominating and disruptive 3techs can be. Even Marvin Lewis, who Del Rio coached under at Baltimore and who loved using 2 big two gapping DTs, fully understands and embraces the disruptive (albeit sometimes undersized) 3tech dynamo(see Geno Atkins).
Just because Del Rio (and others) are scheme diverse, does not mean they pass on a once in a generation type player, because they were forced to use the resources at hand (2 bigger guys who fairly beat out all the other guys, smaller or faster notwithstanding). And FYI, Bears wanted to re-sign Melton, and not as a back up.